The present invention generally relates to an implant to be used for internal fixation of bone fragments to effect osteosynthesis of ununited bone fragments. The implant is particularly suitable for fractures of long bones although it may be adaptable to bridge joints of long bones requiring arthrodesis or bones in a state of non union, malunion or pseudoarthrosis.
It is well known in the art of bone fixation that the repair of fractured bones may be accomplished by the attachment of bone plates and intramedullary nails or rods to the injured bone to hold the fractured bone ends in place during healing. Bone plates and intramedullary rods or nails are designed to provide rigid fixation and support for applied loads while being subjected to cyclical loads in tension, compression, torsion and or bending.
Bone plates are generally described as devices with at least one flattened surface and with holes or grooves for screws and wires, respectively, situated in or along the main body of the plate, to allow fixation of the flattened surface of the device to the bone surface by means of screws or wires for the purpose of holding the bone in place and achieve union of the bone fragments.
The bone plates traditionally are rigidly fixed to the bone to prevent motion between the fragments. Empirically bone union occurs with rigid fixation, but rigid fixation of the bone fragments along significant lengths and breaths of the bone will weaken the bone through stress shielding and disuse atrophy. The adverse effects of stress sheilding and disuse atrophy are prolonged healing time and refracture or discontinuity of the bone if the device is removed after osteosynthesis.
Other adverse effects of plates are as follows: Healing also is generally without the formation and protective function of external callus; applying the plate for rigid fixation requires surgical dissection of the non-osseous tissues, a process which injures the external vascular and nutritional sources of the bone fragments and which may be imprudent in the presence of preceding traumatic injury to the bone and non osseous tissue.
Screw holes of plates are weak points or stress risers that may cause failure or breakage of the plate during load application especially if this load application is repetitive or cyclical. If a plate does not have fixation applied throughout its entire length, fixation may be inadequate for load support during load application which is usually in several planes, a situation that may result in loss of axial and rotational alignment, malunion or nonunion of the fragments and or failure of the device along the stress rising non utilized screw holes. Needless to say, if several screw holes are left unused then the remaining portion of the plate is usually not rigid enough to withstand cyclical applied loads without failure or deformation.
Generally, the loading configuration to which an implant is subjected is not limited to one particular plane. There may be simultaneous forces in several planes. If this is the case, cross sections which are asymmetrical may not be as satisfactory as those which are symmetrical for load bearing purposes. Thus, a plate which is usually flattened on one or more surfaces will not bear loads equally in all directions and may be adequate to withstand forces in one direction but inadequate to withstand forces in another. By comparison a round section device has equal properties for load distribution and bearing in all directions.
Intramedullary nails or rods are commonly used to support long bone fragments to effect osteosynthesis. The rod has several advantages over the plate. Placement can be subcutaneous at an entry point to the intramedullary canal of the long bone thereby avoiding surgical injury to the extra osseous tissues that provide nutritional and vascular support to the bone fragments especially in times of injury thereby lessening the risk of infection. Unlike plates, they share functional loading in weight bearing during and after the osteosynthesis process thereby preventing disuse atrophy as seen with plate fixation for osteosynthesis. This feature makes a second operation for removal to allow functional load distribution to the bone often unnecessary. If removal is necessary, refracture of the bone is uncommon, unlike the case with removal of plates, because the functional capacity for load bearing returns to the bone during and after healing and before removal, since the rod shares function with the bone to which it is applied, thereby avoiding stress shielding of the bone.
For intramedullary osteosynthesis of long bones, the rod or nail may be rigid, flexible, circular, diamond shaped, rectangular, of open section or closed section. However, it has been proven that for a given cross sectional area, a closed circular configuration with symmetry in all directions is most reliable in sustaining forces applied in several planes.
The intramedullary rod or nail conventionally applied, has several disadvantages. Insertion technique has a steep learning curve and can be technically demanding and requires expensive and sophisticated equipment and well trained support personnel. Positioning of the patient must be precise to allow proper insertion and this is not always possible or practical for a seriously multiple injured or obese patient. The use of the intramedullary rod or nail is limited, almost precisely to treating the diaphyseal section of the long bone needing osteosynthesis.
Although axial alignment is usually assured with intramedullary rods or nails, rotational alignment is not assured unless the rod has a fluted end or unless the rod is locked proximally or distally with screws, a procedure that is difficult to do in the distal locking area. Because of the great difficulty in achieving precise screw placement, this step usually prolongs the operative time and time of exposure to radiation, consequently, intramedullary rodding or, nailing must be performed using fluoroscopy, to ensure precise placement.
In general, application of a strong, rigid rod for intramedullary placement for osteosynthesis of a long bone requires intramedullary reaming, a process that entirely destroys the inner ⅔ of the intramedullary vascular circulation to the diaphysis of the long bone. The outer ⅓ of the diaphysis is supplied by the external non osseous tissue. If this is also disrupted by injury at the time of reaming for nail or rod insertion then the undesirable situation of the diaphysis being completely without vascular supply exists making the bone fragments more susceptible to infection or the chances of union more unlikely.
If the rod is placed without reaming then the constraints of the intramedullary canal limits the diameter size of the rod or nail, a situation that may make it too thin and flexible for effective load bearing or support such as seen in cyclical weight bearing.
Moreover, after reduction of the fragments, the osteosynthetic device must be rigid enough to hold the fragments in the restored position and alignment during load application especially for the long bones of the lower extremity engaged in the cyclical load bearing of walking and for the long bones of the upper extremity engaged in cyclical load support as seen in crutch walking, for example.
The designer should make the device sufficiently rigid so as to provide no more than the maximal tolerable amount of relative motion during the healing process. Controlled motion at the non-united ends of the long bones is desirable to stimulate callus formation. The implant should also be rigid enough to withstand load sharing forces in all planes (compression, bending, twisting and tension), but not so rigid as to force the implant to continuously carry the load after healing has taken place since this situation would lead to fatigue failure of the implant. On the other hand, too much motion from a pliable or flexible rod could lead to a hypertrophic non-union in a long bone.
Considering the variation in anatomy and the biologic constraints on size of the device, the ideal osteosynthetic implant is difficult to select by material selection criteria only. However, in selecting an ideal device attention must be paid to factors including the combination of design, application, material selection, selection of cross sectional areas and lengths in broad categories such as small, medium and large. The device should meet ideals of minimal soft tissue damage during application, rapid application with very limited amount and use of sophisticated equipment and personnel, load sharing with the bone fragments to which the device is applied, before and after osteosynthesis. In addition, the device will provide support for the rapid development of external callus driven by the stimulus of load sharing that causes controlled, benign motion at the ununited bone ends; rigidity and rigid fixation away from the bone ends that will allow controlled motion at the ununited bone ends while at the same time allowing load bearing and support, even of a cyclical nature. Further, the invention describes an implant for osteosynthesis that will initially bear the total load of the injured biologic structure, since the initial and basic purpose of this device, should be to provide a means of load transmission across fractures or ununited bone fragments before synthesis has been achieved.
In accordance with applicant""s invention, it is possible to overcome the many defects attributable to intramedullary rodding and extramedullary bone plate fixation through the use of an extramedullary rod capable of being rigidly attached to the extremities of a long bone, provide weight bearing support to the bone along its longtudinal axis and permit stimulatory forces of motion that generate callus repair at the point of nonunion of the bone. An extramedullary rod of this type is unknown to those skilled in the art.
A principal object of this invention is to provide rigid fixation of the bone fragments that will maintain axial and rotational alignment during load bearing of osteosynthesis.
Another object of this invention is to provide a bone fixateur that is preshaped to accommodate the general anatomy of the bone fragments to which it is applied and restore normal or near normal axial and rotational alignment of the bone after union.
Yet another object of the invention is to allow functional load sharing throughout the fixation before osteosynthesis and after osteosynthesis.
A further object of the invention is for the device to be applied in a rigid manner to the bone fragments with minimal surgical damage to the soft tissues that are external and internal to the bone.
One more object of the invention is for it to be applied with little or no contact of the rod section to the bone fragments while still providing rigid support to the fragments for load bearing.
A final object of the invention is by design and application to provide a method that would allow beneficial motion at the ununited bone ends of long bones that will stimulate the formation of external bridging callus between the ununited bone ends.